Train Valley [Steam store page]
Starts out simple, but to be really good at this game you have to be able to multi-task incredibly accurately and quickly. That just isn't me, but the game is well designed and will be fun for people who love the challenge of optimizing their strategy and reaction times to climb the leaderboard.

Needs a faster fast forward, and has a few annoyances if you're trying to get Expert achievements - but the normal gameplay is fun and well designed.